When a power supply connected to an electronic device is turned on, an initial current is provided by the power supply to the electronic device. This initial current may come in a surge that is much larger than the current which flows in a steady-state operation of said device. Such a surge of current is known as an inrush current, and may lead to a tripping of a protection device supporting the electronic device or permanent damage to electronic components of the electronic device.
Inrush current to electronic devices can be limited by providing a resistive component between the power supply and the electronic device. An example of such a component is a negative temperature co-efficient (NTC) thermistor. The resistance of a NTC thermistor decreases as the temperature of the NTC thermistor is raised. By providing one or more NTC thermistors in series between the current supply and the electronic device, a resistance is provided to reduce the current flowing into the electronic device. When the device is first switched on, the NTC thermistors are ‘cold’ and thus impede the inrush current and limit the effect of said inrush current on the electronic device. This resistance will significantly reduce as the NTC thermistors are heated by the current flowing through them, thus providing limited resistance to the final, steady-state current flowing to the device.
A disadvantage of employing NTC thermistors as a means for inrush current protection is that, after the current flowing through them is removed, the NTC thermistors remain ‘warm’ with a low resistance. Thus if the current is turned off and then on again before the NTC thermistors have been able to cool to a high resistance state, there will be limited or no protection provided to protect against any inrush current associated with this turn on. Sometimes the current surge will be modest or absent, in which case the equipment will continue as if nothing has happened, but on occasion the surge will be very large and may cause a protection device to trip.
One solution to this problem is to divert the supply of current away from the NTC thermistors following a period immediately after switch on of the power supply in which the inrush current occurs (the inrush period). The NTC thermistors, with no current flowing through them, are then able to cool down in preparation for a subsequent switch on and resultant inrush current surge. For example, a timing circuit may be provided that engages a switch that short circuits the NTC thermistors after a preset time period. Once the supply is turned off, the timing circuit loses power and subsequently the switch is reset, allowing for inrush current protection for a subsequent turn on of the power supply.
However, in such a circuit the switch of such a timing circuit is not disengaged immediately upon turning off of the supply. This is a consequence of residual charge retained within the circuit (for example in capacitors employed in the timing circuitry or in reservoir capacitors employed to smooth the input voltage of the timing circuit), which takes time to discharge. Thus the components within the timing circuit are still subject to a driving potential during this decay period, including the switch. The switch will therefore disconnect after a time dependent on the decay of the residual charge contained in the circuit. Thus following the turn off of the supply, there is a short time period before the switch resets. While the switch reset time period is significantly shorter than the cool-down time of the NTC thermistors, this can still be problematic if there is a reconnection of the power supply within this period. This is because while the NTC thermistors may be in a ‘cold’, high resistance state, they are still short-circuited and therefore not arranged to limit any inrush current. Thus in this short time period the circuitry is vulnerable to inrush currents.
A large surge of inrush current may occur, for example, in devices including a separation transformer, where momentary interruptions cause the transformer to drop out and then power up again, causing the core of the transformer to become saturated as the supply is reconnected. In a similar situation, large inrush currents may occur if the mains plug is plugged into the device but the connection is not cleanly made, i.e. there are a series of momentary connections, with each one causing the thermistors to heat up and reducing their effectiveness. If the protection device were to trip under these conditions, it may not be able to reset until some time afterwards, when the thermistors have cooled down.
Consequently, there is a need to provide for inrush current protection in the event of brief interruptions of the power supply.